Adjustable frequency generators having voltage-controlled oscillators are known. For instance, in the book by U. Tietze and Ch. Schenk "Halbleiter-Schaltungstechnik" (Semiconductor Circuit Technology), 4th ed., 1978, pages 440 and 441, a voltage-controlled oscillator is described, the output frequency of which is proportional to the input voltage. Not only this known voltage-controlled oscillator, but oscillators of this type generally have a nonlinear control characteristic; this means that the frequency changes nonlinearly with the input variable. It is therefore necessary to calibrate the voltage-controlled oscillators.
This also applies to an adjustable frequency generator such as is described in U.S. Pat. No. 4,103,250. This known frequency generator contains a voltage-controlled oscillator which is preceded via a digital-to-analog converter by a ROM which may be part of a computer. Upon the selection of a frequency, the oscillator is acted upon via the ROM, taking into consideration the nonlinear characteristic of the voltage-controlled oscillator by means of the subsequent digital-to-analog converter, by an input voltage such that it generates the selected frequency on the output side. By means of a RAM and an electronic supplemental device, the accuracy of the frequency generation can be improved.